1. Field of the Invention
The present invention is directed to a process or method for producing a gas discharge lamp and particularly a flash tube and at least two electrodes which are connected in a gas tight fashion by an intermediate glass at the ends of the glass tube.
2. Prior Art
A gas discharge lamp which is used for a flash tube is disclosed in an article by C. Meyer, "Recent Developments In Electronic-Flash Lamps", Philips Technical Review, Vol. 22, 1960/61, No. 12, pages 377-390. The flash tube such as disclosed in this article in the simplest circumstances may consist of a straight piece of glass tube which has an electrode fused in a gas tight fashion at each end so that an anode is disposed adjacent one end and a cathode is at the other end. Generally, the anode consists of tungsten or molybdenum and the cathode consists of a sintered body which comprises saturating substances that are composed of emission materials and getter materials which are well known and described, for example, in German printed patent AS 23 32 588. The discharge tube or lamp is filled with an inert gas preferably xenon on account of its spectral light distribution, which is similar to natural daylight. An ignition or triggering electrode is generally located on the outside of the tube.
To initiate the gas discharge, the ignition or triggering electrode initiates the gas discharge between itself and the cathode by producing an electrical field which raises as rapidly as possible and, therefore, the gas adjacent the cathode becomes ionized due to the effects of the field and causes a gas discharge to take place. This gas discharge will extend in the direction of the anode until the field strength of the electrical field prevailing between the cathode and the anode becomes of such a magnitude due to displacement of the part of the gas which has not become ionized that the remaining gas is also ionized. Consequently, the main gas discharge between the cathode and anode is triggered. Initiation of the gas discharge can also take place without a separate ignition or triggering electrode if a so-called "overhead ignition" occurs in which the anode receives an adequate voltage pulse.
The glass tube which serves as a discharge vessel consists of quartz crystal glass or hard glass having a very high melting point. The electrode material or at least the material of the metal or metallic electrode connector pins, which passes through a gas tight seal of the glass tube and extends to the actual electrode arranged inside the glass tube, must be selected to be such that the different coefficients of thermal expansion between the material of the electrical connector pin and the glass tube do not lead to cracks in the gas tight connection or seal. When hard glass is used for the glass tube, this matching can be effected by selecting tungsten for the electrodes or at least for the portion of the electrical connector pin extending through the glass envelope and by matching the coefficient of thermal expansion of the tungsten with a hard glass of appropriate composition. It should be noted that matched glass of this type is commercially available.
In case of a quartz crystal glass, a direct matching is not possible. In this instance, as in the case when a hard glass is used in fact for the glass tube, but for economic reasons and primarily to reduce cost, nickel is used as the lead instead of the more expensive tungsten, a transition element composed of an intermediate glass must be provided in order to match different coefficients of thermal expansion.
Although tungsten in combination with a matched hard glass has an advantage in comparison with other metals that no intermediate glass is required, the cost of tungsten is relatively high and tungsten cannot be soldered. A compromise of using expensive metal, which can sustain a high thermal load, only for the actual electrodes, of employing a sintered body for the cathode, and of producing the electrical connector pins for the two electrodes from a cheap metal necessitates utilizing an intermediate glass, which results in an equally expensive solution due to the high cost of the process steps which are required.
In forming the known flash tubes, the first step was sealing the electrical connector pins or supply lines for each electrode in an intermediate glass. For the next stage, two possible processes were available. As disclosed in the above mentioned article from Philips Technical Review, the electrode along with its supply line or connector pin, which is in the intermediate glass and serves to support the electrode, is sealed in the opposite end of the glass tube by the immediate glass to the tube, which is provided with its own pump connection. After the sealing operation, the glass tube is evacuated through its own pump connection, subjected to a degassing process, and then filled with a filling or inert gas to the required pressure. After the filling operation, the connection is subsequently fused closed.
One of the other possible ways of forming the tube comprises securing one of the electrodes together with its supply line or connector pin in one end of the glass tube with the sealing forming a gas tight seal, then a second electrode seal in the intermediate glass is positioned in the other end of the tube and the securing of this second electrode is combined with the processes of the evacuation, degasification, and filling and closing of the glass tube. In this case with a straight glass tube, this type of process has many advantages compared to the first mentioned process which required the provision of a separate pump connection of the glass tube.
When using this second process, the second electrode is provided with its supply line or connector pin in a sealed fashion in a so-called glass hose of intermediate glass to form a unit and this unit is inserted into the open end of the glass tube forming the tube of the assembly. The glass tube is substantially longer than the desired final length of the gas discharge lamp. In order to fix the position of the second electrode at a point at which the seal is to be made, the glass tube is slightly impressed or indented by heating this point. Thus, the unit containing the second electrode is loosely received in the glass tube but cannot drop out. The next portion of the process involves subjecting the assembly with one electrode secured and closing one end and the other electrode being in a unit freely received in the glass tube to an evacuation process, followed by a degasification process and then followed by a filling process of the glass tube with the desired inert gas. Each of these processes of evacuation, degasification and filling take place through the open end of the glass tube. After the filling process, a final sealing of the unit comprising the electrode and its supply line is carried out by further heating of the glass tube at the place of the indentation to form the final seal of the end. After forming this second seal at the second end, the excess and superfluous end of the glass tube, which is an excess of the desired length, is cut off, and the step of cutting can be done in one operation with the sealing and closing of the end of the tube.
This second process has the advantage that the separate pump connection is not necessary because one end of the glass tube serves automatically for this purpose; however, the process does have the disadvantage that quite a lot of glass is wasted. Another disadvantage resides in the fact that the exact position of the second electrode cannot be fixed within close tolerances. Therefore, a desired spacing between the two electrodes cannot be accurately achieved. In addition to these disadvantages, both of the above mentioned processes have the disadvantage that the securing of the electrode feed lines with the intermediate glass involves expensive glass blowing operations.